Method of regenerating sodium
sulfite waste liquors



United States Patent 3,248,169 METHQD L REGENERZZ'ATING SGDIUM SULFITEWASTE LIQUORS Nils Viktor Mannhro, Box 349, ll iarrnaverlren, weden NoDrawing. Filed Aug. 12, 1957, Ser. No. 677,755 Claims priority,application Sweden, Aug. 15, 1956, 7,546/56 13 @lairns. (Cl. 23-48) F orthe manufacture of sulfite cellulose pulp from cellulosic materials, anumber of pulping processes utilizing sodium sulfite cooking liquorshave been proposed. Mainly it has been a question of an equivalent ofthe conventional sulfite pulping with calcium sulfite cooking liquor insuch a manner that the calcium base has been replaced by sodium base.Due to the higher solubility of the sodium sulfites, a greater freedomof choice as regards the composition of the cooking acid has beenobtainable and in addition to cooking acids comprising NaI-ISO and S0 ithas been possible with advantage to use only NaHSO or Na SO Furthermorepulping to a high yield or semichemical pulp has been carried out withsuch cooking liquors or pulping has been carried out as a neutralsulfite pulping with cooking liquors comprising Na SO and Na CO o1 NaHCOAdvantageously, the pulping has been carried out in several stages.

Pulping with sodium suffite cooking liquors enhances the yield as Wellas the quality of the pulp. Compared, e.g., to calcium sulfite pulps,the sodium sulfite pulps are more readily bleached. In spite of theseadvantages the sodium sulfite pulping has not attained widespread use.This is due to the fact that the economically necessary regeneration ofthe spent cooking liquor chemicals has proved to be too complicated.Only in some mills, neutral sulfite pulping t-o semichemical pulp hasachieved a more extensive use, since under these special circumstancesit has been considered possible to let the relatively small amount ofcooking chemicals go to waste.

In recent years, several sodium sulfite liquor regeneration processeshave been described in the technical literature. Most of them involvecombustion of the sodium sulfite spent liquor in liquor combustionfurnaces or socalled soda house units in a manner similar to thetreatment of the black liquor of the sulfate pulping process. In thecombustion of the sodium sulfite liquor there is obtained a soda smeltwhich consists mainly of Na CO Na S and smaller amounts of other sodiumcompounds such as Na SO Na sO and Na S O It is usually suggested that asolution of the soda smelt be carbonated as a Whole with CO or cO-containing gases. Thereby, sodium sulfide (Na s) is converted intohydrogen sulfide and sodium bicarbonate (NaHCO By burning the hydrogensulfide to sulfur dioxide and reacting it with sodium carbonate, sodiumsulfite is obtained. This quantity of sodium sulfide, however, is onlyabout half of the total amount of sulfur supplied to the soda house unitwith the sulfite spent liquor. The remainder of the sulfur leaves withthe combustion gases, mainly as sulfur dioxide. Recovery of the sulfurdioxide content of the combustion gases, therefore, would be importantto the economy of the process. A circumstance preventing a completesulfur recovery is the fact that sodium sulfide and sulfur dioxide orbisulfite together form sodium thiosulfate (Na S O which may becomeenriched to the point where the pulping is jeopardized or renderedwholly impossible. This will be the case, if carbonation is not completeand carbonated solution is used to scrub out sulfur dioxide from theflue gases. Complicated systems are described according to whichcarbonation is rendered complete by using carbon dioxide gas produced inthe process. These processes also involve driving off hydrogen sulfidegas 3,248,169 Patented Apr. 26, 1966 from the carbonation as soconcentrated a gas that it can be burnt to sulfur dioxide in a specialfurnace. These processes involving the handling of carbon dioxide andhydrogen sulfide gases necessitate a complex equipment.

In other processes, it has been suggested to allow sodium carbonate tocrystallize out of the solution of soda smelt and then oxidize sodiumsulfide to sodium sulfite. In one process, a slurry of concentratedsodium sulfide solution and sodium carbonate crystals is oxidized withair under specific conditions which require a meticulous control. Thereaction product comprising mainly sodium sulfite and sodium carbonateis dissolved and utilized to scrub out sulfur dioxide from the liquorcombustion gases. A similar crystallization of sodium carbonate incombination with carbonation has also been suggested in connection witha combination of the sodium sulfite and sulfate processes (N. Mannbro,Svensk Papperstidning, 5 8, 1956, 525,571). According to a recentpublication (R. A. Nugent and R. Q. Boyer, Paper Trade Journal, 140,1956, Nos. 18 and 19) this crystallization of sodium carbonate withsimultaneous addition of a soda smelt comprising Na S and Na CO requiresa special procedure. It is possible to crystallize out a pure soda froma solution of molten soda of normal concentration, if the solution iscooled under certain specified conditions. This process, however, is noteconomically attractive, wherefore it has been attempted to crystallizeout the soda from a solution saturated in Na CO by adding furtherquantities of Na S and Na CO as a soda smelt. However, the soda smeltsolidifies in the solution to lumps and Na CO cannot be recoveredwithout entraining too much sodium sulfide. According to the last-citedpublication, this problem has been solved by atomizing the soda smelt,before dissolution thereof, by means of a steam jet similar to theproduction of mineral wool. The atomized soda smelt then forms sodiumcarbonate crystals which after suitable separation may be washedentirely or substantially free of sodium sulfide in a centrifugalapparatus or on a filter. In accordance with a process developed in thisconnection, the Na s-enriched mother liquor is admixed with sulfiteliquor recovered from the digesters. Thereby, the waste liquor is madealkaline and the Na CO added precipitates calcium derived inter aliafrom wood and water as calcium carbonate which is removed. The alkalizedsulfite liquor is oxidized by passing air therethrough, whereby thesulfide sulfur added is converted into Na S O Thereby, release ofsulfidic sulfur as hydrogen sulfide during evaporation is prevented. Inthe liquor combustion the sulfur of the sulfite liquor is reduced sothat approximately one-half thereof is removed from the soda house unitas sodium sulfide in the soda smelt. Furthermore, about one-half of thesulfur leaves with the combustion gases as sulfur dioxide and isrecovered by scrubbing the flue gases with a solution prepared from thecrystallized soda, The same process has also been modified so that thesoda is crystallized by evaporating the solution. The evaporationequipment used, which is of the salt evaporator type, may be connected,with respect to the vapor driven off from the solution, to the sulfiteliquor evaporation system. This embodiment may therefore be found morefavorable than the crystallization process above described.

The present invention involves the advantage that the sulfidic sulfur isnot subjected to oxidation to sodium thiosulfate before introductioninto the liquor combustion furnace. Apart from the considerablesimplification that the oxidation equipment can be omitted, this has theadvantage that the sulfur balance is improved. This is explained in thefollowing manner. In the combustion of the liquor, there arises in theresultant smelt an equilibrium which may be determined by thermodynamicprinciples of calculation. In this case it is desirable that the smeltcontains more Na CO and less Na S and other sulfur compounds and theflue gas as much S as possible. If the sulfidic sulfur is oxidized tothiosulfate prior to the liquor combustion, this sulfur will be bound toa greater extent to the soda smelt and once more leave the liquorcombustion furnace in the form of Na s. The greater the amounts ofsulfur compounds circulated within the system of liquor combustion-sodasmelt-sulfide oxidationevaporation and back to the liquor combustion,the less satisfactory are the results achieved both as regards cheruicaland heat economy. If the sulfidic sulfur as such is directly burnt to S0this undesirable reconversion into Na s is prevented in the part of theliquor combustion furnace where a reducing combustion takes place. In aliquor combustion furnace it is possible schematically to distinguish azone where an oxidizing combustion takes place and where the liquor iscarbonated, dried and drydistilled to a material which is transferred toa zone for reducing combustion. is determined by the material suppliedto the reduction zone and the conditions of combustion therein. Afterhydrogen sulfide has been released, it is burnt to S0 under morefavorable conditions than those prev-ailing in the reduction zone. Fromthe flue gas, sulfur dioxide is recovered by known methods which involveseparation of sodium carbonate or sodium bicarbonate from the soda smeltor a solution thereof and the use of such sodium carbonate orbicarbonate for scrubbing the flue gas.

A simple manner of converting the sulfidic sulfur into sulfur dioxidecomprises admixing the Na S-enriched mother liquor with the evaporatedliquor passed to the liquor combustion furnace. In order not to burdenthe sulfite liquor regeneration with the additional quantity of Waterpresent in said mother liquor, it may be of advantage to dissolve thesoda smelt in sulfite liquor of a suitable concentration. In order thatis bisulfite content shall not react with the sulfide to form inter aliathiosulfate this sulfite liquor may initially be neutralized. Suchneutralization may be made with soda recovered in the process, and inthis connection it may be advisable also to wash the Na CO crystals freeof mother liquor with such sulfite liquor. It was found thatcrystallization of Na CO is readily achieved by the use of this process.If the temperature conditions are suitable, a certain amount of watermay be removed as water of crystallization with the soda crystals.

To minimize the quantities of sodium and water carried to the liquorcombustion with the sulfidic sulfur, this may to a greater part or tothe desired extent be converted into hydrogen sulfide. This may be donein a manner known per se by the use of ion exchangers so that the liquoris acidified thereby. However, it appears more simple to carbonize theNa S-enriched mother liquor with CO or CU -containing gases, mostpreferably with combustion gas from the liquor combustion. In this case,combustion gas should first be purified to remove sulfur dioxide. Theprocedure herein described may be carried out in a simple manner Withoutthe need of a special combustion furnace for H 3. H 5 may be burnt at alow concentration and Without hazardous handling, nor will gasometers orthe like be required. Since only a minor part of the combustion gas isrequired therefor, the liquor combustion will not be interfered with, ifhydrogen sulfide is driven off in admixture with flue gas which isreturned to the furnace Where combustion to sulfur dioxide takes place.carbonation will not need to be made complete in this process, nor willany partial oxidation of the sulfide by the oxygen of the flue gas tothiosulfate have any disturbing effect. This is due to the fact that tieresidual solution may be renewed progressively to the extent desired anda continuous conversion of it and its content of indifferent sodiumcompounds may take place by admixture thereof to the sulfite liquorbefore its combustion.

The composition of the soda smelt ssures In this manner, sodium will berecovered as crystals of sodium carbonate and/or sodium bicarbonate. Bykeeping the amount of sodium compounds circulating through the liquorcombustion furnace at a minimum, but maintaining the S compound, arelatively greater amount of S compounds may be removed with the fluegas as sulfur dioxide to be made use of. Methods have also beendeveloped to recover hydrogen sulfide in a concentrated form in spite ofthe use of combustion gases for carbonation (Swedish Patent No.151,066). Hydrogen sulfide recovered by such a method may be burntseparately to sulfur dioxide and in accordance with the presentinvention be absorbed together with sulfur dioxide in the liquorcombustion gases or at a laterstage in a subsequent separate absorptiontower.

From the sodium carbonate which has been crystallized out, a sodasolution of suitable concentration is prepared. For this purpose, wateror suitably a fraction of the dilute sulfite liquor obtained in thewashing of the sulfite pulp may be used as solvent, since recycling ofspent sulphite liquor to the digestcr can be carried out withsatisfactory results in sodium bisulphite pulping in contrast to calciumbisulfite pulping. The latter procedure, unless pulping is carried outat an extremely low temperature, results in black cooks after a numberof recycling steps. Thus, in a sodium sulfite process the solids contentof the waste liquor can be increased in this manner, which is of a greatimportance as regards heat economy. It has also been found that thesulfite liquor content in the absorption liquor acts an an inhibitor andprevents oxidation of sulfite, e.g., to sulfate.

Depending inter alia on the temperature conditions, sulfur trioxide willalso be present in the flue gas in a state of equilibrium with sulfurdioxide. In the sulfur dioxide scrubber, sulfate will therefore beformed and Will be carried over to the cooking liquor. Sulfate may bepermitted to be present in the cooking liquor, but if it is consideredthat it is more useful to have the corresponding amount of chemicals assulfite, it is advisable first to absorb sulfur trioxide from the fluegas. Simultaneously the flue gas is cooled rapidly to 'a range oftemperature where the formation of sulfur trioxide is insignificant. Byrecirculating scrubber liquid, it Will be acidified with sulphuric acidand absorb sulfur trioxide but not sulfur dioxide. The acidic scrubberliquid recovered can be passed to the unevaporated sulfite liquor sothat ultimately the sulfate sulfur is reduced to sulfide sulfur in theliquor combustion furnace. By heat exchange of the scrubber liquid, theheat content of the flue gas may be utilized to heat water or to preheatsulfite cooking liquor. Equipment similar to that used at present insome sulfate mills may be used. As the scrubber liquid, sulfite liquorof a suitable concentration may be used in order not to burden theprocess unnecessarily with added Water.

In a practically closed recovery process, an accumulation of chloridessupplied with the wood, the Water or the chemicals may cause corrosion.By scrubbing the flue gases with an aqueous solution of sulfuric acidand bydrochloric acid recirculating to the desired extent, absorption ofsulfur dioxide in the scrubber liquid will be prevented to such anextent that the part of the scrubber liquid that must be removed fromthe process will contain little sulfur dioxide. To be able toheat-exchange the scrubber liquid without undue corrosion it isnecessary to keep the concentration of chlorides below a certain level.In this connection, one might study the possibility of supplying to thissodium sulfite liquor regeneration process suitable alkaline spentliquors obtained by bleaching or refining pulp. By fractional recoveryof such alkaline spent liquors from a bleaching plant, it would bepossible to discharge the alkaline spent liquor containing the greatestproportion of chlorine compounds separately as a waste product and toutilize the remainder. Further, the possibility may be mentioned ofcarrying out alkaline treatments of pulp in connection with bleaching orother refining with a suitable soda solution obtained from this process.In some methods of preparing chlorine dioxide for bleaching pulp,sulphur dioxide is employed, which can be recovered from this process,and it may also be possible to utilize the scrubber liquid in chlorinedioxide production or acidification of pulp. A combination of suchprocesses in the opposite direction can also be suggested. Thus, in somemethods of producing chlorine dioxide, products are formed which may beutilized in a sodium sulfite liquor regeneration process. Examples areacidic sodium sulfate solutions. In the production of chlorine dioxide,it has moreover been found suitable to add sodium sulfite liquor to thereaction mixture to increase the yield of chlorine dioxide.

The liquor combustion gases also carry with them an alkali dustconsisting mainly of sodium sulfate. This is usually recovered with theaid of a dust separator, e.g., an electrostatic filter prior toscrubbing. But scrubbing to recover sulfur trioxide may also be carriedout in a known apparatus in such a manner that sulfur trioxide andalkali dust are recovered together from the flue dust. The simplest formof this part of the process consists in passing the combustion gasesdirectly from an electrostatic filter to the absorption device wheresulfur dioxide is absorbed in soda solution. In this case, thetemperature of the gas will influence the conditions of absorption. Theusual procedure in the sulfate process consisting in cooling thecombustion gases with simultaneous heating of air for the liquorcombustion, would be applicable.

The crystallized soda is dissolved in the manner set forth above. Whenthe soda crystals are separated from the mother liquor, some impuritiesderived from the soda smelt will accompany them, e.g., carbon particles,iron sulfide, calcium carbonate, etc. These impurities are caused tosettle from the soda liquor and/ or are removed by centrifuging orfiltration. For this purpose, it is suitable to employ the equipmentused in the sulfate process for purification of white liquor. To preventlosses of chemicals in the removal of the sludge, this may be washed inconnection with centrifuging or the like. The Washing liquid may bereturned to the process.

The crystallized soda will contain a minor quantity of sulfide. Thetower for absorption of sulfur dioxide should therefore be constructedso that carbon dioxide in the combustion gas passing through thesolution is permitted by carbonation of the solution to displace a majorportion of this sulfide sulfur as hydrogen sulfide. Since the sodacontains only about a few percent of sodium sulfide and the removal ofsulfide may easily be rendered practically complete (about 90%), thequantity of sodium thiosulfate formed will be insignificant. It willprobably be found unnecessary to contemplate recovery of the smallamount of sulfide sulfur driven off as hydrogen sulfide. However, thepossibility exists of carrying out such a carbonation in a specialtower, using only a part of the SO -free combustion gas, and returninghydrogen sulfide to the liquor combustion furnace or to a scrubber withbisulfite-containing sulfite liquor.

The composition of the sulfite solution obtained from the absorption ofsulfur dioxide depends on the composition of the sulfite liquorsubjected to combustion which in its turn depends on the pulping methodused. In normal sodium bisulfite pulping, additional amounts of sulfurdioxide must be added to obtain a sodium sulfite cooking acid. Since theloss of sulfur dioxide in such a process is relatively greater than thatof sodium, it is necessary, even if the losses in chemicals were to beconvered with sodium sulfate or sodium sulfite instead of sodiumcarbonate or sodium hydroxide, to introduce additional sulfur dioxide inthe process. Usually, pyrite or sulfur will be burnt for this purpose.The sulfur dioxide gas obtained is absorbed in the sulfite solutionfirst obtained to prepare a so-called crude acid. The crude acid isintroduced into the system where cooking acid is .or sodium bisulfiteand sulfur dioxide.

prepared by utilizing sulfur dioxide gas and heat derived from thedigesters. In order not to introduce carbon dioxide into this system,care must be taken that a complete decarbonation of the solutionsintroduced has taken place. Such a decarbonation can be achieved with abisulfite solution, whereby also concentrated carbon dioxide gas isdeveloped.

If it is desired to make use of the heat of the flue gases to thegreatest extent possible, the sulfur dioxide absorption in warm sulfitesolution or crude acid may be conducted under pressure according toknown processes.

The spent liquor obtained from the pulping step will contain minoramounts of calcium derived from the wood and the water, which togetherwith the sulfate content of waste liquor may cause formation ofincrustat-ions or scale during evaporation of the waste liquor.Evaporation equipment has been developed, which overcomes theincrustation problem in the evaporation of conventional calcium sulfitewaste liquor, and such equipment will be suitable, if it is desired toevaporate the acidic waste liquor and possibly make use of the sulfurdioxide containing condensate to prepare fresh cooking liquor. For otherreasons it may be advantageous to evaporate a neutralized 0r alkalizedwaste liquor. If for this purpose a solution derived from the processand containing carbonate or crystallized soda is added to the spentliquor, the calcium content of the spent liquor may be precipitated ascalcium carbonate and/or calcium sulfite prior to evaporation. Such aneutralization may also take place in connection with fermentationprocesses. It was mentioned above, that sodium sulfite pulping may becarried out in one or more stages. In the regeneration process hereindescribed, sodium sulfite, sodium bis'ulfite, sodium carbonate andsodium bicarbonate may be recovered. Furthermore, concentrated sulfurdioxide may be recovered from the systems used in the present sulfitemills for gassing sodium sulfite to sodium bisulfite In sulfite pulpingprocesses, where sulfur dioxide gas is present in the digesters or inthe cooking acid tanks, this sulfur dioxide may be withdrawn and ifdesired be liquefied to form liquid sulfur dioxide which may be injectedinto the digester charge, as desired. 7

In such processes, the cellulosic material is first sulfonated with acooking liquor containing sodium sulfite or sodium bisulfite, whereuponthe cooking liquor is drawn off and in a subsequent hydrolysis stage isreplaced by a cooking liquor containing sulfur dioxide or sodiumbisulfite and sulfur dioxide. An injection of sulfur dioxide gas in thehydrolysis stage is also used. Such multistage pulping processes areused, e.g., for pulping barkdamaged spruce Wood and for pulping pinewood.

Among other pulping processes, the process may be mentioned in which thewood is first sulfonated with a solution of sodium sulfite or sodiumbisulfite, to which solution, sometimes after withdrawal of some cookingliquior, sulfur dioxide is added by injection into the digester charge.After the wood has been subjected to further pulping to the extentdesired and if desired a part of the sulfur dioxide has been removed, asolution of soda smelt or a soda solution is injected into the digestercharge. Pulps suitable for the preparation of cellulose derivatives mayeasily be prepared in this manner, or the lightness, bleachability andyield of sulfite pulpmay be combined with the strength and low contentof extractives of sulfate pulp. In accordance with the regenerationprocess herein described, all the chemicals required for such a processare available, and as an example, the last pulping stage may be effectedwith a solution of soda smelt obtained by dissolving the soda smelt intwo separate dissolving vessels or with a portion of thesulfide-containing mother liquor from the crystallization system. It isalso to be noted that for pulping and bleaching processes it is possibleto prepare sodium hydroxide by causticizing a solution of soda smelt orcarbonate.

A single regeneration process may be utilized simultaneously for severalpulping processes. Thus, it has been suggested to prepare cooking liquorfor neutral sulfite or so-called NSSC pulping from a sulfite spentliquor obtained from a normal sodium bisulfite pulping process (N.Mannbro, Svensk Papperstidning, 55, 1952, 665). Otherwise, the NSSCmethod has the disadvantage that since the pulp is produced in a highyield (semichemical pulp), the amount of combustible substances in thespent liquor is so low that difliculties arise, if the chemicals are tobe regenerated by combustion of the spent liquor. According to thisprocess, it is possible for example to produce at the same time a commonsulfite pulp from spruce and NSSC pulp from birch or wood waste. NSSCpulp has obtained a very widespread use for manufacturing wrappingmaterial. It may also be bleached and used as a substitute for pulpsproduced by normal pulping processes.

Besides the woods above referred to, a great number of hardwoods andsoftwoods as well as other cellulosic materials, such as straw, bamboo,etc., may be utilized in these pulping methods.

By known methods, by-products can be recovered from the sodium sulfiteliquor. Examples of such methods include various fermentation processes,methods of recovering lignosulfonic acid products, e.g., with the aid ofion exchanges and for the production of vanillin or furfural. In somecases, one or more of the chemicals available in the regenerationprocess herein described may be employed in such methods or converselychemicals obtained in the by-product recovery may be utilized in thisprocess.

The invention is illustrated by the following example.

Example With the sulfite spent liquor to the recovery furnace 1162kilograms of solids were supplied per metric ton of pulp. Besides waterand wood substance dissolved during digestion, this sulfite liquor hasthe following composition:

Kilogram-mols Na O 2.84 S as sulfite and lignosulfonic acids 2.04 Na S1.11 Na SO 0.30

The amount of Na S indicated consists of Na S separated from the sodasmelt from the furnace and returned to the sulfite liquor according tothis invention.

Due to the fact that this Na S has been added to the sulfite liquorwithout oxidation to Na S O this sulfidic sulfur is driven off in the CO-container atmosphere in the furnace to about 90% as hydrogen sulfidewhich is burnt to sulfur dioxide which is mixed with the combustiongases. Thereby, 1.0 kilogram-mol of S is formed.

In the carbonizatio-n of the liquor, additionally 1.02 kilogram-mols ofS0 are driven off from sulfite and lignosulfonic acids. Together withthe 80; formed according to the thermodynamically established reactionequilibrium in the soda smelt, totally 2.11 kilogrammols of S0 leavewith the combustion gas.

The combustion gas has the following composition:

Percent S0 0 65 CO 11.2 H O 23.6 N 62.0 0 2.6

After passage through a heat recovery and dust separation system, thecombustion gas is passed to an absorption tower filled with packing to aheight of 4 meters. The absorption liquid consists of 0.5 molar sodasolution with a temperature of about 60 C. In the lower portion of thetower, the absorption liquid is recirculated and to the upper portionthereof soda solution is supplied.

8 More than 96% of the sulfur dioxide in the combustion gas is absorbedin this tower.

The soda smelt from the furnace has the following compositionMol-percent Na CO 52.8 N328 41.3 Na SO and other substances calculatedas Na SO 5.9

The soda smelt is dissolved in a concentrated solution of sodium sulfideat a temperature of 05 C. This solution contains about 40 g. of Na C0per 1000 g. of water and 580 g. of Na S per 1000 g. of water and some NaSO In the dissolution of smelt soda in this solution carbonate crystalsare for-med. The solution is maintained at the concentration referred toby continuously drawing off mother liquid from the carbonatecrystallization. With this mother liquor, the total amount of Na S ofthe smelt is returned to the recovery furnace by admixture to sulfitespent liquor which is evaporated so that the mixture contains 56% ofsolids with the chemicals contents indicated above.

Carbonate crystals in an amount corresponding to 1.46 kilogram-mols ofNa CO per metric ton of pulp are recovered, accompanied by about 0.04kilogram-mol of Na SO Thus, there are obtained from the absorption towerfor the combustion gas per metric ton of pulp:

Kilogram-mols S0 2.0 Na O 1.46

Kilogram-mols S0 3 .5 N320 1 .46

To compensate for the losses of chemicals, 24 kg. of Na SO and 16 kg. ofsulfur are added per metric ton of pulp. The sulfate is added to theevaporated sulfite liquor before combustion thereof. The sulfur is burntto S0 which is absorbed in the solution from the absorption tower.

I claim:

1. A method of regenerating chemicals from spent sulfite liquor obtainedin pulping of oellulosic materials and producing a spent liquorcontaining sulfite compounds which are substantially preserved as suchand which by subsequent combustion give off sulfur dioxide to thecombustion gases which comprises evaporating the sulfite spent liquor inthe presence of its original sulfite sulfur compounds and burning saidsulfite liquor in a liquor combustion furnace to form soda smelt andsulfur dioxde-containing combustion gases, preparing a solution of saidsoda smelt, containing predominantly sodium carbonate and sodiumsulfide, causing sodium carbonate to crystallize out from said solutionby using such a concentration of soda smelt that the solubility of thesodium carbonate is exceeded, separating said sodium carbonate from themother liquor, passing the sulfidic sulfur in the Na senriched motherliquor at original oxidation state to the liquor combustion furnacewhere the major portion of said sulfidic sulfur as intermediate hydrogensulfide is burnt to sulfur dioxide, and absorbing said sulfur dioxide aswell as sulfur dioxide present in said liquor combustion gases in asolution prepared from the crystallized sodium carbonate to form a freshsulfite cooking liquor.

2. A method as in claim 1, in which said soda smelt is dissolved in onefraction of the sulfite spent liquor, separating sodium carbonate andpassing the mother liquor with sulfidic sulfur to the liquor combustionfurnace.

.3. A method as in claim 13., in which the Na s-enriched mother liquoris added to suliite liquor, passing the mixed liquor with the sulfideand sulfite compounds at substantially their original state of oxidationto the liquor combustion furnace.

4. A method as in claim 3, in which the Na s-enriched mother liquor isadded to evaporated sulfitc liquor and is passed to the liquorcombustion furnace.

5. A method as in claim 1, in which the Na S-enriched mother liquor isacidified by carbonating it with a member of the group consisting ofcarbon dioxide and carbon dioxide-containing gases so that hydrogensulfide is lib-erated and said hydrogen sulfide is burnt to sulfurdioxide in the liquor combustion furnace.

6. A method as in ciai-m 5 in which sodium bicarbonate formed in thecarbonation is crystallized out and separated from the mother liquor.

7. A method as in claim 5, which comprises passing to the liquorcombustion a portion of the mother liquor required for a continuousremoval of indifferent sodium compounds enriched in the mother liquor.

8. A method as in claim 1 in which a compound from the group consistingof sodium hydroxide, odium suliite, and sodium carbonate is added to thesolution of recovered sodium carbonate.

9. A method as in claim 8, in which the combustion gases, prior to theabsorption of sulfur dioxide, are scrubbed With an aqueous solutionsuitably sulfite liquor to be burnt, to cool the combustion gase andremove sulfur trioxide therefrom leaving the sulfur dioxide for saidabsorption.

iii. A method as in claim 9 in which alkali dust in the combustion gasesis separated dry prior to the scrubbing step and is returned to theprocess.

11. A method as in claim 9, in which a minor portion of the scrubberliquid is passed to the liquor combustion stage and the major portion isrecycled to the scrubbing stage.

1.2. A method as in claim 9 in which the circulating liquor is cooled byheat exchange with a liquid stream to the pulping process.

13. A method of treating spent sulfite liquor obtained from cookingcellulosic materials with a suliite cooking liquor which comprises thesteps of:

evaporating Water from said spent liquor to form a concentrated liquor;

burning the concentrated liquor in a combustion furnace to form a sulfurdioxide containing combustion gas a smelt containing sodium and sulfur;

dissolving the smelt to obtain a solution containing sodium suliide andsodium carbonate; precipitating the sodium carbonate and separating thesodium carbonate from the solution of sodium sulfide;

passing the sulfide sulfur content of said solution to the combustionfurnace Without appreciable reaction or": said sulfur content with thesulfur in the spent liquor wherein a portion of the sulfur so passed isconverted to sulfur dioxide;

and absorbing the sulfur dioxide so produced in a solution prepared fromthe precipitated sodium carbonate.

llieierences Cited by the Examiner UNiTED STATES PATENTS MAURICE A.BRENDISI, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

13. A METHOD OF TREATING SPENT SULFITE LIQUOR OBTAINED FROM COOKINGCELLULOSE MATERIALS WITH A SULFITE COOKING LIQUOR WHICH COMPRISES THESTEPS OF: EVAPORATING WATER FROM SAID SPENT LIQUOR TO FORM ACONCENTRATING LIQUOR; BURNING THE CONCENTRATED LIQUOR IN A COMBUSTIONFURNACE TO FORM A SULFUR DIOXIDE CONTAINING COMBUSTION GAS AND A SMELTCONTAINING SODIUM AND SULFUR; DISSOLVING THE SMELT TO OBTAIN A SOLUTIONCONTAINING SODIUM SULFIDE AND SODIUM CARBONATE; PRECIPITATING THE SODIUMCARBONATE AND SEPARATING THE SODIUM CARABONATE FROM THE SOLUTION OFSODIUM SULFIDE; PASSING THE SULFIDE SULFUR CONTENT OF SAID SOLUTION TOTHE COMBUSTION FURNACE WITHOUT APPRECIABLE REACTION OF SAID SULFURCONTENT WITH THE SULFUR IN THE SPENT LIQUOR WHEREIN A PORTION OF THESULFUR SO PASSED IS CONVERTED TO SULFUR DIOXIDE; AND ABSORBING THESULFUR DIOXIDE SO PRODUCED IN A SOLUTION PREPARED FROM THE PRECIPITATEDSODIUM CARBONATE.